Fuel cells have the great advantages that there is in a substantial sense no need for the fossil fuel which requires to pay attention to the depletion of resource, no or little noise is produced upon power generation, and the recovery of energy is high as compared with other energy power generation devices. The construction of fuel cells as relatively small size power generation plants installed in buildings and factories has been under development and some are used in practice. In particular, polymer electrolyte fuel cells operate at low temperature as compared with other types of fuel cell. They eliminate in a material aspect a concern about the corrosion of cell constituent parts and can discharge a relatively high current flow for low-temperature operation. They are thus of great interest not only as household cogeneration devices, but also as substitute power supplies for internal combustion engines on vehicles. The polymer electrolyte fuel cells are constructed of several components including a separator which is generally in the form of a plate having a plurality of parallel channels in one or both surfaces thereof. The separator plays the roles of conducting to the exterior the electricity generated on the gas diffusing electrode within the fuel cell, discharging the water formed in the channels during the electricity generation process and maintaining the channels as a flowpath for reactant gas to flow into the fuel cell. The fuel-cell separators are required to be reduced in size. Additionally, since a plurality of separators are used in stack, there is a need for a separator sealing material which remains fully durable in long-term service.
As the separator sealing material, packing materials based on various resins have been under study in the art. Sealing materials based on silicone rubber are often employed for their moldability, heat resistance and elasticity. Of the silicone rubbers, cured rubbers of addition curing type silicone rubber compositions have been used because of their ease of molding, but are still unsatisfactory in maintaining elasticity over a long term. In particular, such cured rubbers are difficult to satisfy both acid resistance and seal performance in acidic aqueous solution which are requisite as the packing material for fuel-cell separators.